A medical system includes an acquisition unit configured to acquire a tomographic image of an object to be examined based on a combined beam generated by combining a return beam returning from the object to be examined illuminated with a measuring beam and a reference beam corresponding to the measuring beam, a focus position changing unit configured to change a focus position on the object to be examined;an optical-path-length difference changing unit configured to change a difference in optical path lengths between the measuring beam and the reference beam, and a control unit configured to control the optical-path-length difference changing unit so that the difference in the optical path length is within a predetermined range and then receive an instruction to the focus position changing unit to make a change.